1. Field of the Invention
The present invention relates to a display device and to a display unit. More specifically, the present invention relates to a display device which may be used as a display of a personal computer, a mobile phone, a mobile terminal, etc., or which may be used as an independent portable display, such as a digital (or electronic) paper or a digital book that obtains information from such appliances. Also, the present invention relates to a display unit which forms such a display device.
2. Description of Related Art
Display devices such as that shown in FIG. 5 are conventionally known as disclosed in Japanese Examined Patent Application, Second Publication, No. Sho 50-15115. In the display device shown in FIG. 5, glass plates 1 and 2, at least one of which is optically transparent, are disposed so as to oppose each other via wall members 3 with a predetermined distance therebetween, and thereby an enclosed space 4 is formed by the glass plates 1 and 2 and the wall members 3. Also, transparent electrodes 5 and 6 of a flat shape made of, for instance, an indium-tin oxide (ITO) are fixed on the inner surface of the glass plates 1 and 2, respectively. Moreover, display liquid 7 is filled in the enclosed space 4. The display liquid 7 includes a colored dispersion medium 7a which may be colored black, for instance, and white charged particles 7b dispersed in the dispersion medium 7a. 
In the above display device, when positive voltage is applied to the upper electrode 5 and negative voltage is applied to the lower electrode 6 as shown in FIG. 6A, the above-mentioned white charged particles 7b, which have been negatively charged, electrophoretically move towards the anode due to Coulomb force, and attach to the surface of the anode (i.e., the upper electrode) 5. When the display device in this state is viewed from a position indicated by the arrow shown in FIG. 6A, portions on which the white charged particles 7b have attached and have formed a layer thereof appear white through the transparent electrode 5 and the glass plate 1. On the other hand, if the polarity of the applied voltage is reversed, the white charged particles 7b attach to the electrode 6 on the other side and form a layer thereon as shown in FIG. 6B. Accordingly, when the display device in this state is viewed from a position indicated by the arrow shown in FIG. 6B, the display surface looks black since the layer of the white particles 7b is hidden by the black dispersion medium 7a. In this device, once the white charged particles 7b attach to either of the electrodes, it is not necessary to apply the voltage to the electrode other than maintaining the attached state of the charged particles 7b. 
Since the above-mentioned display device is of a light reflecting type, an image formed by the display device is kind to the eye in the same way as printed matter, and does not cause eyestrain due to flickering of emitted light.
However, in the display device explained above, a problem of uneven contrast may be caused due to the aggregation of the charged particles (for instance, zinc sulfide) 7b or the attachment of the particles to the electrodes, etc. In order to solve this problem, another type of display devices have been proposed which includes, in an enclosed space formed between electrodes, a plurality of spherical microcapsules in which charged particles are dispersed in a dispersion medium, and binding material used for filling the gap among the microcapsules as disclosed in Japanese Unexamined Patent Application, First Publication, No. Sho 64-86116 and U.S. Pat. No. 6,017,584. According to the display device, the above-mentioned problem of uneven contrast can be solved, and the resolution of the display may be improved.
However, the display device explained above also has problems derived from using a technique to form microcapsules. That is, in a procedure for synthesizing microcapsules, it is difficult to obtain microcapsules with a uniform size distribution. If the sizes of the microcapsules are not uniformly distributed, it becomes difficult to obtain a display layer of uniform thickness using the microcapsules. Also, if the sizes of the microcapsules are not even, the distance the charged particles migrate in each of the microcapsules becomes different. This means that the response time necessary for switching displays becomes different for each of the microcapsules.
On the other hand, since it is difficult to obtain microcapsules of uniform size during a synthesizing process, it may be possible to select only microcapsules of the same size using a filtering process, for example, after synthesizing the microcapsules. However, in this case, other problems, such as damage to the microcapsules while passing through the filter, may be caused.
This fragility of the microcapsules is problematic not only during the manufacturing process thereof, but also after the microcapsules are made into a product and are used by a user. For example, when a display device is formed on a flexible plate to be used as an electronic paper, the microcapsules may be damaged when the plate is bent, and the display liquid may leak from the damaged portions.